Deep in lakes and muddy sediments, certain bacteria navigate using an internal compass made of iron crystals.
For decades, scientists have known these microbes align with Earth’s magnetic field – but no one had directly measured how strong that tiny biological magnet really was.
.0Now, researchers have done exactly that. For the first time, scientists have directly measured the mag0netic strength of a single bacterium’s internal compass, revealing how firmly it aligns with Earth’s field and how much force it takes to override it.
In doing so, they have transformed a long-observed natural behavior into hard numbers that engineers can finally design around.
Testing the cell’s internal compass
Mounted on a hair-thin flexible beam, one Magnetospirillum gryphiswaldense cell revealed the full pull of its internal magnetic chain.
By tracking subtle shifts in the beam’s motion, a team at the University of Basel documented how the bacterium’s magnets aligned and resisted external fields.
Those measurements showed that the cell’s magnetic chain remained stable under natural conditions yet responded predictably as fields intensified.
That stability sets a clear boundary between reliable biological navigation and the stronger magnetic forces required for deliberate external control.
A compass built for oxygen
Within certain bacteria, a line of iron-rich crystals gives the whole cell a preferred magnetic direction. Biologists call these structures magnetosomes – membrane pockets that grow iron minerals into magnets – and the crystals usually form a chain.
During swimming, that chain turns the cell until its body points along the magnetic field it senses, reducing wasted spins. That simple physical push explains why researchers find these microbes in lakes and wet sediments where oxygen levels change by depth.
In muddy water columns, oxygen can drop fast, and these bacteria survive best in a narrow middle zone.
Magnetic alignment reduces sideways wandering, helping bacteria reach their preferred oxygen level with fewer wasted turns. Without that cue, swimming becomes more random, and the microbe spends extra energy sampling layers that do not suit it.
Because Earth’s magnetic field stays steady, the compass provides a reliable shortcut in habitats where chemical signals are patchy.
Measuring the faint magnetic pull
Even sensitive instruments can miss this signal, so Mathias Claus, a doctoral student at the University of Basel, needed a different approach. To boost sensitivity, the team used a cantilever – a tiny beam that bends and vibrates easily – as a magnetic sensor.
“We first attached a single bacterium to an extremely thin cantilever and measured its vibrations in magnetic fields,” said Claus.
Tiny shifts in vibration revealed how strongly the chain resisted being twisted away from its chosen direction. Instead of measuring force directly, the bacterium’s attempt to rotate added a measurable twisting pull on the beam.
High-resolution imaging matched those vibration patterns, allowing researchers to estimate the chain’s full magnetic strength.
That cross-check mattered because the signal was so weak that even small errors could have misrepresented the cell’s true behavior.
Pushing the magnet too far
Under natural conditions, the magnet chain carries enough strength to line the bacterium up with Earth’s field on its own.
In water and sediment, that alignment turns random swimming into directed motion, and the single-cell measurements showed the chain could resist small environmental jolts.
But beyond a certain field strength, the internal chain stops behaving predictably and the bacterium loses its preferred direction. At those extremes, Dr. Boris Gross at the University of Basel tested control fields relevant to magnetic microrobots.
“This is an important aspect for potential technical applications, such as controllable microrobots,” said Dr. Gross.
When the field direction reversed, small clusters of magnets inside the chain sometimes snapped into the opposite orientation. simulations – computer models that track how tiny magnets flip – confirmed those jumps.
Because the bacterium was fixed during tests, it could not rotate freely, so the chain absorbed the stress. In nature, Earth’s field is far weaker, and a free-swimming cell simply turns smoothly until its direction settles again.
Guiding microbes on purpose
Engineers want to steer these microbes on purpose, because a self-powered swimmer can carry cargo where pumps cannot reach.
In mice, a 2016 study guided bacteria that follow magnetic fields while carrying drug-filled capsules, and up to 55 percent reached low-oxygen tumor regions.
Elsewhere, another paper showed that a strain of bacteria that naturally grows tiny internal magnets could remove toxic chromium from water, and those magnets allowed researchers to pull the cells back out with a simple external magnet.
Those early demonstrations depend on predictable magnetic behavior. Now, single-cell magnet readings turn what once looked like a biological navigation trick into a set of measurable numbers that engineers can design around.
Instead of guessing how strongly a bacterium will respond, researchers can define what control systems can realistically and safely demand.
Future work will need to test more living cells in real fluids, where rotation, turbulence, and crowding effects may change the rules.
But with precise measurements in hand, magnet-guided microbes move one step closer to becoming practical tools for medicine and environmental cleanup.
The study is published in the journal Physical Review E.
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